Dengue viruses belong to the flavivirus group whose members also include yellow fever, Japanese B encephalitis, Russian spring-summer encephalitis, and other less characterized but highly pathogenic viruses. Dengue infection is usually manifested by fever, severe debility and bone pain; however, mortality is rare except in th dengue hemorrhagic fever-shock syndrome that occurs in children. A successful immunization procedure for prevention of dengue infection has not been developed. Dengue virus type 4, the most recent isolate associated with epidemics in the Western hemisphere, was chosen to initiate our molecular biology study. Virion RLA extracted from virus preparations was analyzed by electrophoresis on formaldehyde-agarose gels. A majority of vRNA appeared to be full-length migrating as a single band. To reverse-transcribe the dengue vRNA, we first carried out polyadenlyation at the 3 feet-terminus using the E. coli poly A addition enzyme. The poly A-dengue RNA will be used for reverse-transcription using oligo (dT) as a primer. The single-stranded complementary DNA transcripts will be converted to DNA duplexs for cloning in E. coli.